As summer approaches, our suburban Minnesota yard is filled with birds. Many are old friends that remain year-round, eking out a living through the winter and fledging their young in our backyard by summer. Others fly hundreds or thousands of miles seasonally to access the best resources, which--it never ceases to amaze me--are found right here.

Of all these stunning songsters, few can outshine the indigo bunting. This year, the male bunting has appeared at my feeder almost every day. Sadly, this living jewel rarely sticks around long enough for photo ops. He is compelled to sing, and every few minutes heads off in search of a high perch. Encoded within the bunting's songs--which are learned in his first year of life by listening to neighboring males--are dozens of notes and complex phrases that broadcast his species and perhaps even his individuality, warn intruders from his territory, and attract unmated females. Perhaps even more than food, song sustains his life.

As you might surmise, the indigo bunting is named for his brilliant appearance. Researchers have been surprised to find the variety of means by which coloration is achieved in birds. Compare the feathers of an indigo bunting to those of a northern cardinal under the microscope. A cardinal's feather contains pigments. These red, orange, and yellow chemicals primarily come from the bird's food. You might have noticed that male and female cardinals are sexually dimorphic--they have different coloration. That's due in part to male cardinals' ability to convert certain dietary pigments to new forms, producing a distinctively bold red shading.

By contrast, you'll find no blue pigments in the feathers of indigo buntings. Instead, they contain a dark pigment called melanin. As light passes through the feather's many tiny barbs, short-wavelength (blue) light is scattered by air spaces surrounding the melanin granules. The exact shade of blue you see varies--from shimmering azure to deep ultramarine--depending on lighting conditions and the angle at which you view the bird. It's like seeing an old friend in beautiful new clothing at every meeting.

Observations of this phenomenon--called structural coloration--date back hundreds of years. Microscopist Robert Hooke and physicist Isaac Newton (in 1665 and 1704, respectively) independently published the results from their microscope studies of peacock feathers. Each noted the way "thin plated bodies" in the feathers interacted with light.

You can see evidence of structural coloration among other blue-shaded species such as bluebirds, blue jays, and kingfishers. This also causes the iridescence of hummingbirds. Nanoparticles (what Hooke referred to as "tiny plated bodies") in the feathers have even been identified in bird fossils approximately 50 million years old.